EAST ARKANSAS VALLEY. 191 



than at any other point, and it is not improbable that this is the laccolite 

 from which the rest of the body has spread out. The rock of this porphyry 

 mass (72) is a fresh-looking, light-gray rock, containing large pinkish 

 crystals of orthoclase, abundant quartz (showing generally a crystalline 

 form), and small hexagonal leaves of biotite. The groundmass is gray and 

 subordinate to the crystalline constituents in quantity. Under the micro- 

 scope it is seen that nearly half of the smaller feldspars are triclinic and 

 much altered, while the larger ones are comparatively fresh. This por- 

 phyry is as nearly the equivalent of the Lincoln as of the Eagle River 

 type, and is one link in the chain of evidence showing that all these allied 

 types constitute one large group. (See Appendix A.) 



At the base of the Dome is an outcrop of quartzite dipping to the 

 southeast, which rises as one follows the cliff southward. In the little 

 ravine next south is a second body of porphyry, separated from the main 

 sheet by quartzites and shales through which it penetrates somewhat irreg- 

 ularly; it may be an offshoot from the main body, though it differs some- 

 what lithologically and is moreover impregnated with secondary pyrite; as 

 it is very much decomposed, its character cannot be definitely determined. 

 At the mouth of Bird's Eye gulch the porphyry body has risen to a con- 

 siderable height on the ridge; while below it, between the mouth of Bird's 

 Eye gulch and Indiana gulch, is a body of the finer-grained dioritic-looking 

 porphyry already mentioned, which crosses over into the bed of Indiana 

 gulch higher up, in the direction of the dike of the same rock on Prospect 

 Mountain Ridge. 



The slopes of Mosquito Range between Bird's Eye and English gulches, 

 east of the porphyry body, are mostly made up of sandstones and occasional 

 beds of black shale of the Weber Grits formation, whose prevailing dip is 

 10 to 15 a little to the south of east. On the summit of the ridge between 

 the head of Bird's Eye gulch and the Arkansas Valley, however, the beds 

 have a shallow dip to the west, giving evidence of a slight synclinal roll, as 

 has been indicated in Section B. 



On the west face of the ridge separating the head of Bird's Eye gulch 

 from that of English gulch is an outcrop of limestone, which is probably 

 one of the beds that occur in the middle of the Weber Grits series. Asso- 



